How Much Phosphorus Do We Want in Our Soils?

Phosphorus is one of 17 chemical elements that all plants need, and it’s one of the nutrients that we sometimes need to add to soils in fairly high quantities. Phosphorus levels in soils depend on the ancestry of the soil and how it’s been managed during its farming history. Phosphorus availability in soils is very strongly associated with pH. At low pHs, phosphorus tends to bind up with iron and aluminum in soils and becomes unavailable to plants. At high pHs, phosphorus can bind to calcium and magnesium and that also decreases it’s availability. Acidic soils bind up phosphorus worse than alkaline soils do. Phosphorus is most readily available to plants at pHs of at least 6.5. If the soil pH drops below 6.0, phosphorus becomes very unavailable. Applying lime to very acidic soils is always a good idea, and one of the benefits of that is to help phosphorus become more available even without adding it as a supplement. There are several methods used by various soil testing labs for determining how much phosphorus soils can supply to the crops we grow in them. When we interpret the results of soil tests, we need to consider the method used by the laboratory that does the test. The numbers that the various tests provide don’t really mean anything on their own until they are correlated to how well crops grow at the various levels. Some of the common extractions for phosphorus are the weak and strong Bray solutions (Bray-1 and Bray-2), Mehlich-1 and Mehlich-2, the Morg

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